Dust deposition as a nutrient source for fynbos ecosystems, South Africa

Fynbos vegetation is the main component of the Cape Floristic Region that occurs in areas of the southwestern Cape of South Africa. A major question concerning the fynbos ecosystem is how it supports abundant and diverse vegetation on generally nutrient-poor bedrock. As part of a general study of the geochemistry of fynbos ecosystems, dust deposition was found to be the likely source for clay particles in the soil and represent a significant source of nutrients (e.g. K, Al, Ca) to the ecosystem. Mountain fynbos, in particular, occurs in areas underlain by Peninsula Formation sandstone, which is a quartz arenite with greater than 98 wt% SiO2. Elandskloof is a headwater catchment in the Cederberg Mountains with a fynbos ecosystem that is entirely underlain by the Peninsula Formation. The soils in Elandskloof are sandy with an organic carbon content of 1 to 2 wt%. Analysis of the soil mineralogy by X-ray diffraction shows that sand and silt fractions are almost entirely comprised of quartz. The clay-size fraction constitutes approximately 6 wt% of the soil and is dominated by kaolinite. Elandskloof is a headwater catchment and, therefore, the kaolinite could only have come from two sources: the underlying sandstones or the deposition of dust particles. The minor amount of feldspar and clay minerals in the bedrock (0.5 wt% Al2O3) suggests an aeolian source for much of the kaolinite in the soil. In addition, the isotope composition of soluble lead indicates a mixture of terrigenous and anthropogenic sources, most likely from washout of aerosols and dust. In addition to soil mineralogy, evidence for aeolian contributions to fynbos ecosystems comes from the washout effect seen in the chemistry of rainwater samples collected in discrete time intervals throughout a given storm, as well as in the comparison of electrical conductivity with rainfall rates. Over a two-day period (19-21 May 2002), a "berg" wind event was observed in Cape Town in which visibility was reduced by wind blown dust. These winds, occurring a few times each year, originate on the elevated, semi-arid Karoo region of central South Africa, and travel southwest across the Cape Fold Belt where the mountain fynbos ecosystems thrive. The suspended dust of these berg wind events are washed out with the passing of the next winter front and thereby provide an important source of nutrients that, once introduced, the fynbos ecosystem is highly effective in retaining.